1. Field of the Invention
This invention relates generally to automatic test equipment, and, more particularly, to switching topologies used for routing signals in automatic test systems.
2. Description of Related Art
Automatic systems for testing electronics devices and assemblies commonly use switching matrices for routing electrical signals. These systems often contain numerous tester resources that can exchange electrical signals with a unit under test, or “UUT.” Tester resources come in many different forms, and may include, for example, power supplies, voltage sources, current sources, waveform generators, meters, sampling circuits, and time measurement circuits. The UUT can take different forms as well, including, for example, semiconductor wafers, packaged semiconductor chips, hybrid assemblies, unloaded circuit boards, and circuit board assemblies.
A switching matrix is commonly placed between the tester resources and the UUT to flexibly connect tester resources to I/O terminals of the UUT (e.g., leads, pins, or connector terminals). Ideally, the matrix is flexible enough to connect any of the tester resources to any of the I/O terminals of the UUT. The matrix is also ideally flexible enough to connect any tester resource to any other tester resource. This latter capability allows tester resources to calibrate and/or test one another via signal loop-back.
FIGS. 1 and 2 show a conventional switching matrix 100 for electronic test systems. The matrix 100, often called a “full-crosspoint” matrix, includes intersecting conductive paths organized in rows and columns with a switch at each intersection point. Tester resources can be connected to the matrix at nodes A-N, and I/O terminals of UUTs can be connected at nodes 1-M. Because each row of the matrix 100 intersects each column, any tester resource A-N can be connected to any terminal 1-M of the UUT simply by closing the switch at the intersection point.
FIG. 2 shows how the matrix 100 can be controlled to make particular connections. For example, by closing switch A1, Tester Resource A is connected to UUT Terminal 1. Similarly, closing switch C2 connects Tester Resource C to UUT Terminal 2. To avoid multiple connections, all other switches in the row and column to be connected (e.g., row C and column 2 in the second example above) must remain open. The matrix 100 also allows different tester resources to be connected together, as shown by the connection of Tester Resources D to Tester Resource N via the closures of switches D3 and N3.
Although the matrix 100 is flexible, it can suffer from certain drawbacks. For instance, the matrix 100 has a switch at every intersection point. The number of switches thus grows exponentially with the size of the matrix (i.e., N2 switches are needed for an N-by-N matrix). Switches used at interfaces of automatic test systems are generally mechanical relays. These relays require significant space, and large numbers of these relays tend to crowd space in the tester and particularly around the UUT, where it is often desirable to place other equipment. In addition, mechanical relays tend to be less reliable than other electronic components. Large numbers of relays tend to increase the likelihood that some relays will fail, and thus lowers the MTBF (Mean Time Between Failures) of the overall test system.
Another drawback of the matrix 100 is its introduction of transmission line stubs, which impair the ability of the matrix to pass high-frequency signals without distortion. As shown in FIG. 2, the connection of Test Resource A to UUT Terminal 1 leaves one stub extending from switch A1 to switch N1 and another stub extending from switch A1 to switch AM. These stubs distort signals passing between Test Resource A and UUT Terminal 1 by introducing impedance changes and reflections. The faster the signal, the more it will be distorted by stubs of any particular length.
Stubs lengths can be reduced in the matrix 100 by providing additional switches. For instance, a switch can be added immediately to the right of switch A1, which can be opened whenever A1 is closed, thereby reducing the stub running from A1 to AM. The added switch can be kept closed at other times. Similarly, a switch can be added immediately below A1 to reduce the stub running from A1 to N1. Although remedial measures like this can be taken, they tend to increase the number of relays required, and thus tend to yield diminishing returns.